Studies on the Nature of the Mullite Formed from Co-precipitate of Alumina and Silica Gels

Mullite is studied using co-precipitated gels obtained from mixed silicon ethylate and aluminium ethylate. Mullite is formed at higher temperatures by the thermal decomposition of the spinel-type phase. In the temperature range 1000°-1200°C mullite exhibites twodimensional grain growth in accordance with the equation dα/dt=Kα(1-α), where t=time, α=amount of mullite. An activation energy of 11-12kcal/mol is obtained. At temperatures higher than 1200°C, mullite shows a remarkable grain growth.The lattice parameters a and c of mullite decrease, and the parameter b increases, when the temperature of heat treatment is increased. Also a is increased and b is decreased as the Al2O3/SiO2 ratio increases beyond 3:2. The c parameter shows little change with Al2O3/SiO2 ratio in the range 3:2 to 2:1. The time variations of a and c are thought to be due mainly to the chemical composition, and the time variation of b due mainly to a process of ordering of the lattice. The diagram of c/a-a/b shows clearly the influence of chemical composition, and suggests the possible existence of mullite containing more silica than 3:2.According to the electron-microscopic studies, mullite showed always a needle-like morphology irrespective of the mode of formation.